1. Field of the Invention
The present invention relates to a rubber-reinforced styrene transparent resin composition having well-balanced and excellent transparency, color tone, impact resistance, stiffness, and chemical resistance. The present invention also relates to a method of efficiently producing the resin composition.
2. Description of the Related Art
Rubber-reinforced styrene transparent thermoplastic resins using rubber-containing graft copolymers, which are obtained by graft-copolymerizing a vinyl monomer with a rubber polymer such as diene rubber or the like, have excellent impact resistance, moldability, appearance, etc., and are thus widely used for various applications such as OA equipment, household electrical goods, general merchandise, etc. Examples of the vinyl monomer include aromatic vinyl compounds such as styrene, xcex1-methyl styrene, and the like; unsaturated carboxylic acid esters such as methyl methacrylate and the like; vinyl cyanides such as acrylonitrile, methacrylonitrile, and the like.
In recent years, the rubber-reinforced styrene transparent thermoplastic resins have been increasingly requested to have an improved physical property balance between impact resistance and stiffness while maintaining excellent transparency and color tone, and many attempts have been made so far. However, in the present situation, a satisfactory level is not yet achieved.
For example, Japanese Unexamined Patent Publication Nos. 60-120734, 61-43643, 62-164745 and 4-320441 disclose methods for improving physical properties such as impact resistant, etc. by copolymerizing an acid component such as methacrylic acid or the like with a matrix resin. However, the resultant resin compositions have the problems of poor transparency and color tone because the amount of an acid component is excessively large.
Also, Japanese Unexamined Patent Publication Nos. 7-292205 and 8-134298 disclose that a graft copolymer is added to a melted matrix resin obtained by continuous bulk polymerization to produce an ABS resin having an excellent color tone and an excellent physical property balance between impact resistance and stiffness. However, these publications do not disclose application to high-transparency resin compositions, and in application of the method proposed in the publications to the high-transparency resin compositions, the effect on the color tone and the physical property balance between impact resistance and stiffness is not yet satisfactory.
In the present situation, application of such a transparent ABS resin is restricted due to the low resistance to chemicals such as an organic solvent, and solvents such as a detergent, and the like.
As a means for improving the chemical resistance of a general ABS resin without transparency, it is generally known to increase the content of a vinyl cyanide compound, and various so-called highly nitrile-containing thermoplastic resin compositions are proposed.
Resin compositions generally known from the viewpoint of improvement in chemical resistance include resin compositions (Japanese Unexamined Patent Publication Nos. 4-258619 and 5-78428) in which the graft ratio of a graft copolymer is defined, highly nitrile-containing thermoplastic resin compositions (Japanese Unexamined Patent Publication No. 4-126756) in which a matrix component comprises a methacrylate ester as an essential component, and the like.
However, the above-described conventional highly nitrile-containing thermoplastic resin compositions have different rates of reaction between an aromatic vinyl compound and a vinyl cyanide compound, thereby causing difficulties in obtaining polymers having a homogeneous composition. Therefore, the thermoplastic resin compositions each comprising a copolymer composed of an aromatic vinyl compound and a vinyl cyanide compound are easily yellowed during molding to cause the problem of deteriorating quality due to discoloration.
In this way, the technique of improving chemical resistance by increasing the nitrile content easily causes the problem of discoloration and deterioration in transparency due to an increase in the nitrile content. Therefore, application of the technique of increasing the nitrile content to transparent ABS is conventionally thought to cause fatal defects in transparent resin products.
Therefore, the highly nitrile-containing thermoplastic resin compositions having excellent and balanced transparency, chemical resistance and color stability have not yet been obtained.
A known method of producing a thermoplastic resin composition represented by an ABS resin, which is obtained by mixing a vinyl copolymer and a graft copolymer, comprises polymerizing a vinyl monomer to produce each of the vinyl copolymer and the graft copolymer, and then mixing both polymers by heat melting. An example of the mixing method comprises mixing the vinyl copolymer and the graft copolymer and then melting the mixture by using the melt blending apparatus shown in FIG. 3.
FIG. 3 is a schematic longitudinal sectional view showing an example of a melting twin-screw blending apparatus for blending the vinyl copolymer and the graft copolymer. In this apparatus, the vinyl copolymer and the graft copolymer are quantitatively supplied to a blending hopper 9 through a feed hoppers 7 and 8, respectively. A resin powder obtained by blending in the blending hopper 9 is supplied to a melt blending apparatus 10 at a constant rate, and kneaded and transferred under heat-melting in a system which is heated to a predetermined temperature (230xc2x0 C. or more) required for melt blending. After the elapse of a predetermined time, the product is discharged from a discharge port 6 at a constant rate.
In the apparatus, from the viewpoint of temperature and time, the vinyl copolymer and the graft copolymer receive the same thermal history, and thus excessive thermal history is applied to the graft copolymer to cause the fault that the color deteriorates.
On the other hand, some methods are proposed to decrease the thermal history, in which a graft copolymer is continuously added to and mixed with an as-polymerized copolymer (A) in a melt state to produce a thermoplastic resin composition in a process of producing a vinyl copolymer by continuous bulk polymerization (for example, Japanese Unexamined Patent Publication Nos. 7-292205 and 8-134298).
However, a balance between a color tone and impact resistance is not easily obtained only by the method disclosed in these publications, and further improvement is desired earnestly for stably producing a resin composition having excellent total quality.
Furthermore, some methods are proposed for improving the balance between a color tone, impact resistance, stiffness, etc. and productivity of a rubber reinforced thermoplastic resin, and improving the transparency of a rubber reinforced thermoplastic resin having transparency, in which a highly rubber-containing polymer is added to and mixed with a melted copolymer in a continuous bulk polymerization or continuous solution polymerization process (Patent Application No. 3109378, Japanese Unexamined Patent Publication No. 2000-178405).
This method can improve dispersibility of the highly rubber-containing polymer in the rubber-reinforced thermoplastic resin. However, particularly in production of a low-viscosity type having low shear force in a melt kneading zone in an extruder, the appearance is defective due to poor dispersibility of the highly rubber-containing polymer, and the physical property balance between impact resistance, stiffness, etc. is slightly insufficient in some cases.
Therefore, a rubber-reinforced thermoplastic resin having an excellent balance between appearance, color tone, impact resistance, stiffness, etc., and excellent transparency has not yet been obtained.
An object of the present invention is to resolve the disadvantages of the conventional techniques and provide a rubber-reinforced styrene resin composition having well-balanced and excellent transparency, color tone, impact resistance, stiffness and chemical resistance, and a method of efficiently producing the same.
The present invention provides a rubber-reinforced styrene transparent resin composition comprising a styrene copolymer reinforced with a rubber polymer, wherein the monomer composition of an acetone soluble resin component contained in the resin composition comprises 5 to 70% by weight of aromatic vinyl monomer (a1), 30 to 95% by weight of unsaturated carboxylic acid alkyl ester monomer (a2), 0 to 50% by weight of vinyl cyanide monomer (a3), and 0 to 50% by weight of another monomer copolymerizable with these monomers, and the acid value of the acetone soluble resin component is 0.01 to 1 mgKOH/g.